1. Field of the Invention
This invention relates to electrical contacts that are treated to maintain minimal contact resistance.
2. Discussion of the Known Art
Electrical contacts are generally made from copper or copper alloys due to their relatively high electrical conductivity. Copper alloys oxidize easily, however, which reduces the integrity of their electrical contacts. Therefore, copper electrical contacts are usually coated with a layer of material that oxidizes less readily than copper. One example of such a material is tin, which is typically applied as a coating ranging in thickness from about 0.0001 to about 0.0003 inch. In addition to preventing the copper contacts from oxidizing and thereby maintaining the electrical integrity of the contacts, the tin coating also imparts solderability if needed for the application.
One problem associated with using tin coating is due to the relatively high rate of diffusibility of copper in tin (0.8xc3x9710xe2x88x926 cm2/sec @ 500K). Copper also forms solid solutions with tin, and may also form stable intermetallics such as Cu3Sn and Cu6Sn5, which severely degrade contact resistance, leading to failure of soldered joints or contacts.
To minimize or eliminate the interaction of the copper with the tin, an intermediate, or barrier layer is sometimes applied between the copper layer and the tin layer. Examples of such barrier layers include nickel, palladium-cobalt, and gold. Palladium-cobalt and gold barrier layers are effective but expensive and their use is generally limited to critical connectors for computer applications. Nickel layers are less expensive and are therefore used in high-volume price sensitive applications, such as automotive electronics applications. Ever increasing use of automotive electronics under the hood, which are generally exposed to temperatures of greater than about 100xc2x0 C., have created the need for an alternate barrier layer with superior performance and reduced cost.
Attempts to provide alternative barrier layers include the electroplating of nickel over a nickel-phosphorus layer, as shown in FIG. 1; the use of cobalt-tungsten phosphide has also been reported as a barrier material; and use of a thicker tin layer has also been tried as a way to maintain the electrical integrity of the contacts.
However, thicker tin layers tend to gall, thereby increasing the contact insertion force. xe2x80x9cGall,xe2x80x9d or xe2x80x9cgallingxe2x80x9d as used herein, means plastic deformation at the interface of two surfaces resulting from the two surfaces sliding against each other, retarding further movement. In electrical contact applications, soft tin coated connectors tend to gall when inserted in tin coated female adaptors, thereby increasing the insertion force.
According to the invention, a method of treating an electrical contact member made of copper or a copper alloy includes electroplating a barrier layer on a contact surface of the member wherein the barrier layer is selected from the group consisting of cobalt, cobalt-nickel alloys, cobalt-tungsten alloys, and cobalt-nickel-tungsten alloys; forming the barrier layer to a thickness in the range of from about 0.00001 inch to about 0.0001 inch, and which thickness is sufficient to prevent the electrical contact resistance of the treated contact member from increasing above a given limit over a given period of time at a given temperature; and coating an outer finish layer over the barrier layer, wherein the finish layer is selected from the group consisting of tin, gold, palladium, platinum, silver, and alloys thereof, so that the given limit of electrical contact resistance of the treated contact member is about 10 milliohms at 100 grams contact force, the given period of time is at least 1000 hours, and the given temperature is at least 150 degrees C.